The present invention relates in general to memory devices and in particular to memory devices having conductive lines buried in isolation regions.
Dynamic random access memory (DRAM) memory has enjoyed popular success over other types of memory technology because of its low cost and simple memory cell layout, which promotes scalability. A DRAM memory cell is capable of storing one bit of information and is constructed using only one memory cell transistor and one memory cell capacitor. As such, this memory cell is often referred to as a one-transistor one-capacitor (1T1C) cell. In a typical memory device, collections of 1T1C memory cells are grouped together by bit lines and word lines forming a memory array.
The industry is continually striving to produce DRAM memory devices that provide increased storage capacity, yet provide comparable to improved operational performance. Increase in circuit density is often the result of an ability to manufacture a given device in a smaller physical space than previously possible, allowing an increase in packing density. However, device density in DRAM memory is limited by both the resolution capability of available photolithographic equipment (feature size) and the area consumed by each memory cell in a given memory array.
One known DRAM memory device stacks storage capacitors above memory cells. For example, memory cells are fabricated by forming word line gate stacks over a semiconductor substrate. Bit lines are subsequently fabricated by forming a metal line in a passivating insulating layer over the semiconductor substrate. A bit line is electrically coupled to an associated memory cell by forming a via that passes through one or more layers of the memory device to a bit line contact on the semiconductor substrate. Capacitor structures are also formed over the semiconductor substrate and are electrically coupled an associated memory cell by forming a via through one or more semiconductor layers. As such, for each memory cell, one via is required to connect the bit line to the memory cell and a second via is required to connect the capacitor structure to the memory cell. Under such an arrangement, the capacitor is typically stacked over the bit line. However, this requires a relatively deep via having contact openings that are difficult to form and hard to fill. For example, poor step coverage and adhesion may occur resulting in poor electrical performance and increased contact resistance. Accordingly, there is a continuing need for improved memory.
The amount of charge that a storage capacitor can store is generally related to the amount of storage node surface area. As DRAM dimensions grow smaller, there is an ever-increasing need to maintain storage capacitance values despite more tightly packed circuits. However, the minimum realizable area of the vias required to connect the bit line and capacitor to the memory cell is limited by the minimum realizable feature size. Accordingly, for a given area, space that would otherwise be available for capacitor structures is required to provide the bit line and bit line contacts. As packing density in DRAM structures increases, the via size thus serves to limit the area available for capacitor structures. Accordingly, there is a continuing need for improved or alternative memory device structures.